The invention relates to an insulator with a molding made of ceramic and a hydrophobic coating applied to the surface of the molding.
An insulator with a molding made of ceramic is used variously in electrical insulating engineering. For example, such an insulator is used as a component in microelectronics, as an insulating housing for components in power electronics, but also as a high-voltage insulator for routing overhead power lines or for keeping them apart.
A ceramic is understood as meaning a clay ceramic, a porcelain or a steatite. The ceramic is produced from the starting materials kaolin, quartz, clay, alumina and/or feldspar by mixing the same while adding various substances in a subsequent firing or sintering operation.
The versatile use of an insulator with a molding made of ceramic in electrical insulating engineering is attributable to the specific properties of the ceramic or the ceramic material which cannot be achieved by other materials. For instance, a ceramic is distinguished by high dimensional stability, great hardness and mechanical strength, by a high electrical insulating capability, by advantageous dielectric behavior, by a great corrosion resistance as a result of high resistance to chemical influences and by a great resistance to heat and effects of the weather.
In long-term use, an insulator is subject to a greater or lesser degree of superficial soiling, depending on the location at which it is used, which can considerably impair the original insulating characteristics of the clean insulator. Such soiling is caused for example by the depositing of industrial dust or salts or the separating out of dissolved particles during the evaporation of moisture precipitated on the surface. This is referred to as surface pollution.
A fired ceramic is distinguished by relatively high surface roughness. Since a rough surface soils much more quickly than a smooth one, it is known to provide the surface of the ceramic molding of an insulator with a surface glaze in the form of a vitreous melt. It is attempted in this way to achieve a kind of self-cleaning effect, which considerably reduces the soiling tendency of the insulator. However, the production costs are increased considerably by the application of the glaze. Raw materials, pigments and the preparation and application of the glaze to the sometimes complicated geometries of the ceramic moldings represent a considerable cost factor. The application of the glaze, as an additional process step, also increases the wastage from production.
Many times, the application of a smooth glaze to the surface of the ceramic molding is not sufficient to ensure the electrical characteristics of the insulator on a long-term basis. Since even a smooth glaze cannot permanently prevent deposits, the geometry of the ceramic molding must additionally be designed in such a way that the leakage path for a possible discharge current over the surface of the molding is as long as possible. Thus, for example, a high-voltage insulator has a large number of plate-shaped ribs or shields along a cylindrical shank. Allowance is made for the different locations at which it is used by different numbers of shields, differences in shield inclination and/or differences in shield projection. This configuration has the effect that the leakage path between the two poles to be insulated is increased considerably in comparison with a purely cylindrical insulator. The shield configuration in combination with the smooth glaze allows a kind of self-cleaning effect of the surface of the molding to be achieved by the soiling being washed away by rain.
In comparison with a simple form of the insulator, however, every change in the geometry toward an increased leakage distance means extra expenditure in terms of material and production time and consequently an increase in production costs.
Furthermore, it has been found that even a great leakage path for an insulator with a ceramic molding with a glaze is not always adequate to ensure the desired electrical insulating capability over a prolonged period of time under particular operating conditions. For instance, the glazed ceramic molding of an insulator which is used in cases of high surface pollution must be manually cleared of deposits at regular time intervals in order for the functional capability not to be at risk. In addition, the known glazes, consisting of a vitreous melt, display a disadvantageous hydrophilicity of their surface. A film of water which traps the dirt particles on the surface is formed. The surface of the insulator becomes conductive. As a result, so-called discharge currents develop on the moist, soiled surface, increase until there is a flashover and in this way initiate the electrical failure of the insulator.
To solve the problem, it is known from xe2x80x9cElektrotechnische Zeitschriftxe2x80x94Axe2x80x9d, volume 96 (1995), pages 126 to 128, to apply a coating of silicone additionally to the glaze of the ceramic molding. This takes place by applying a silicone paste or a silicone elastomer. Since silicone is hydrophobic, the surface structure of the glaze is changed in such a way that it repels water. This prolongs the operating characteristics of the soiled insulator.
However, a coating of silicone paste is disadvantageously not durable, and must be renewed from time to time, for example when the system is not in operation. In addition, both the necessary silicone paste and the silicone elastomer are expensive.
Furthermore, the publication xe2x80x9cInsulators Glaze Modified by Plasma Processesxe2x80x9d, Tyman, A.; Pospieszna, I.; Iuchniewicz, I.; 9th International Symposium of High Voltage Engineering, Graz, Aug. 28 to Sep. 1, 1995, discloses an insulator with a molding made of ceramic and a glaze applied to the ceramic, with a hydrophobic, plasma-polymer coating being additionally applied for the protection of the glaze from external influences. Disadvantageously the hydrophobicity and durability of the plasma-polymer coating described are strongly dependent on the type of glaze.
It is accordingly an object of the invention to provide an insulator that overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type, with a molding made of a ceramic which has high long-term stability with regard to its electrical insulating capability, in particular when used in a damp and/or dust-containing environment.
With the foregoing and other objects in view there is provided, in accordance with the invention, an insulator having a molding made of a ceramic and a hydrophobic coating applied to the surface of the molding, a plasma polymer being applied directly to the ceramic as the hydrophobic coating.
In other words, the insulator according to the invention is distinguished by the fact that, instead of a hydrophilic glaze, a hydrophobic plasma polymer is applied directly to the ceramic of the molding. The glaze of the surface of the ceramic molding is no longer needed and is therefore omitted. With the glaze omitted, the molded ceramic of the insulator according to the invention can have a rough surface to which the hydrophobic plasma polymer is directly applied.
Accordingly, the insulator according to the invention consists essentially of a molded ceramic having applied to a surface thereof a hydrophobic coating comprising a plasma polymer.
Previous considerations concerning the improvement of the long-term stability of the electrical insulating properties of an insulator with a molding made of a ceramic were aimed at coating the already waterproof surface of the ceramic with a smooth glaze. The glaze as such was considered an indispensable part of the ceramic molding or insulating body because of the better self-cleaning effect intended by it. For further improvement, it was attempted to compensate for the hydrophilic character of the glaze by a hydrophobic coating applied to the glaze.
In a way surprising to a person skilled in the art, the invention now envisages dispensing completely with the glaze of the ceramic molding, and instead applying a plasma polymer directly to the ceramic of the molding as the hydrophobic coating.
The invention proceeds in a first step from the finding that not only a reduction in the roughness but also an increase in the hydrophobicity of the surface of the molding helps to reduce considerably the soiling tendency of the insulator.
Although it is true that a smooth surface soils less than a rough one, a high degree of hydrophobicity of the surface can compensate for the soiling tendency of a rough surface. This is because, precisely in the case of use in a damp environment or outdoors, most deposits on the surface result from dissolved particles when precipitated water evaporates. If the surface of the ceramic molding then has a high degree of hydrophobicity, the water does not adhere to the surface in the first place, but forms beads together with the dissolved particles and drops off. The accumulation of deposits is thus counteracted.
In addition, in outdoor use, dust-containing deposits on a hydrophobic surface are easily washed away by rain, even if the surface is rough. With regard to the soiling tendency, when the insulator is used in damp conditions or outdoors, the hydrophobicity of the surface of the ceramic molding is accordingly able to compensate for the roughness. This of course also applies when the soiled insulator must in any case be cleared of foreign deposits manually, for example with water, acetone or the like. Also, when used in a very salty atmosphere, such as for example near the coast, a hydrophobic surface of the unglazed molding helps the insulator to achieve better long-term electrical characteristics than a hydrophilic glazed surface.
In a further step, it has been recognized that precisely a plasma polymer is outstandingly suitable as a hydrophobic coating which can be applied directly and with good adhesion to the relatively rough surface of an unglazed ceramic. In accordance with this invention, the term xe2x80x9cplasma polymerxe2x80x9d refers to a polymer produced by plasma deposition, which, as distinct from the polymer produced by conventionally chemical means, has a much higher crosslinking density of the individual molecular groups among one another, is not oriented but amorphous and, moreover, has a much higher density. A plasma polymer is distinguished, for example in comparison with a conventional polymer, by broadening of the infrared vibration bands measured by means of IR spectroscopy.
To produce the plasma polymer, a plasma of ionized molecules is ignited in a suitable reactor in a working gas by applying an electrical field or by coupling in microwaves. Under suitable conditions, the plasma polymer is formed in the plasma on the surface of the substrate to be coated by a wide variety of chemical reactions. For the production of a plasma polymer, reference should be made to the article xe2x80x9cAdvances in Basic and Applied Aspects of Microwave Plasma Polymerizationxe2x80x9d, M. R. Wertheimer et al. in Thin Solid Films, No. 115 (1984), pages 109 to 124. For the production of a hydrophobic plasma-polymer coating on an electrical insulator, reference should be made in particular to the German patent application filed at the same time at the German Patent Office with the title xe2x80x9cHerstellungsverfahren fur einen elektrischen Isolatorxe2x80x9d [production process for an electrical insulator] with the internal file reference GR 98 E 8511, the content of which also constitutes part of the present document.
The precise chemical reactions which lead to the depositing of a plasma polymer from the plasma in the working gas are not yet known in detail today. A plasma polymer also cannot be described by specifying a precise chemical composition, since a plasma polymer is specifically distinguished by a large number of very different molecules crosslinked among one another. Therefore, to designate the plasma polymer, those skilled in the art refer to the working gas used, in which the plasma is ignited. If, for example, hexamethyldisiloxane is used as the working gas, the plasma polymer produced from it is referred to as a plasma-polymerized hexamethyldisiloxane. This designation, common among those skilled in the art, is adopted in this document. It is irrelevant for the invention whether the plasma polymer is firmly bonded to the surface of the ceramic as a result of chemical bonds or whether, on account of a very high crosslinking density of its individual molecular groups among one other, it is so stable that a chemical bond with the ceramic is no longer important.
For generating a hydrophobic plasma polymer, it is expedient if the plasma polymer is produced by plasma deposition from a volatile compound having non-polar groups, i.e. a non-polar gas or a gas having non-polar groups. It has been found in accordance with this invention that plasma deposition from the non-polar working gas or working gas having non-polar groups produces a plasma polymer with a not very reactive, i.e. low-energy, surface. Such a surface is highly hydrophobic, i.e. water-repellent.
Favorable working gases are, for example, hydrocarbons. Consequently, methane or acetylene are suitable.
Particularly good hydrophobicity and a high degree of crosslinkage of individual molecular groups are distinguishing features of a plasma polymer in the form of a plasma-polymerized organosilicon or organofluorine compound. On account of the high degree of crosslinkage, such a plasma polymer is extremely stable and protected against outside effects. Such a plasma polymer has a high degree of hardness. For this reason, such a plasma polymer is of great advantage for the hydrophobic coating of the surface of the ceramic molding of the insulator.
It is particularly favorable for the hydrophobicity, hardness and quality of the plasma polymer if the plasma polymer comprises a plasma-polymerized hexamethyldisiloxane, a plasma-polymerized tetraethyl-orthosilicate, a plasma-polymerized vinyltrimethyl-silane, a plasma-polymerized octofluorocyclobutane or a mixture thereof.
In an advantageous refinement of the invention, the coating has a thickness of between 50 nm and 10 xcexcm. In this way, a hard and durable coating of the surface of the ceramic molding is ensured. With such a thickness, the high degree of crosslinking of the individual molecular groups of the plasma polymer among one another reliably ensures that moisture cannot penetrate through the plasma polymer. Even small molecules such as oxygen, hydrogen or carbon dioxide can no longer penetrate through the cluster of molecules of the plasma polymer.
In a further advantageous refinement of the invention, the ceramic of the molding of the insulator is a porcelain, i.e. a silicate ceramic. Such a ceramic is distinguished by high mechanical strength both with respect to compression and with respect to tension and by a good electrical insulating capability. Such a ceramic is therefore used in particular for an insulator which is exposed to high mechanical loads.
For example, such a ceramic is used for a molding of a high-voltage insulator which is used for routing and/or keeping apart overhead lines or train catenary systems. The plasma polymer applied to the surface of the ceramic molding has the effect of improving the operating performance of the insulator even under environmental influences. In regions with surface pollution, a hydrophobically coated insulator is far superior to a glazed, uncoated hydrophilic insulator.
A high-voltage insulator, in particular with a molding made of a porcelain with an added amount of aluminum oxide, with a hydrophobic plasma-polymer coating of the surface of the molding, is used wherever the longest possible service life has to be ensured in cases of surface pollution and damp weather conditions. Even in the case of use under extreme environmental influences, such as for example in coastal regions where there is a high salt content in the ambient air, or close to industrial sites with industrial dust and aggressive gases in the ambient air, such a high-voltage insulator is distinguished by a much longer, maintenance-free service life with regard to its insulating capability in comparison with a conventional high-voltage insulator. On the one hand, the plasma polymer prevents dissolved particles from being deposited from precipitated water, since the water forms beads off before evaporating. On the other hand, the plasma polymer also achieves the effect that the ceramic insulating body, which is the actual means of providing the insulating properties, withstands environmental influences. Precisely in use outdoors, the hydrophobicity additionally achieves the long-term effect that there are fewer foreign deposits, since every time it rains the precipitated dust is reliably washed away by rainwater. The greatest effect, however, is that, even with an already soiled surface of the insulator, its operational reliability is sustained, because, as a result of the hydrophobicity, no conducting layers of foreign material with critical discharge currents can form.
The invention offers the advantage that an insulator with a molding made of a ceramic can dispense entirely with the previously necessary glaze for treating the surface. The required costs for the glaze and its application are no longer incurred. The process for generating a plasma polymer on the surface of a substrate, in particular a ceramic, is substantially known. Apart from the once-only procurement of a plasma reactor with the required other components, the production of a plasma polymer is a relatively low-cost process. An insulator with a molding made of a ceramic with a plasma polymer applied directly to the ceramic can be produced at lower cost, or at least at the same cost, as a conventional insulator with a molding made of a ceramic and a glaze applied to the ceramic. Replacement of the glaze by a hydrophobic plasma polymer has the effect of drastically reducing the risk of flashover as the final consequence of the formation of critical discharge currents. Even in cases of dust deposits, it has been found that, precisely when the insulator is used outdoors, it is possible to compensate for the greater roughness of the surface of the ceramic molding by the hydrophobicity of the plasma polymer. An insulator with a molding made of a ceramic and a plasma polymer applied directly to the ceramic is distinguished by extremely favorable long-term characteristics with regard to its electrical insulating capability. The time between routine cleaning and maintenance operations for systems subject to and at risk from pollution can be drastically extended.
The invention also offers the advantage that it is possible to dispense with a special, complex geometry of the molding to increase the leakage path. Since the hydrophilic glaze is replaced by a hydrophobic plasma polymer, the ceramic insulator is more reliable, specifically under environmental influences.
The depositing of particles when precipitated water evaporates is also avoided.
The invention also allows a significant reduction in the multiplicity of types with regard to the required geometries of the ceramic molding. In an ideal case, the invention allows, for example in the case of a high-voltage insulator, that it can be designed in a substantially cylindrical or rod-shaped form. In this way, it is even possible to achieve the effect that it is impossible for dust deposits to be deposited.
The invention consequently makes possible insulators with a ceramic molding of a relatively simple geometry with at the same time favorable long-term characteristics with regard to the electrical insulating capability. In this way, the material costs for the manufacturer are considerably reduced in comparison with conventional insulators with a complicated geometry. For the user, currently necessary cleaning and maintenance work is no longer needed or becomes necessary at much longer time intervals.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an insulator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.